1. Field of the Invention
This invention relates to the grid of a corona discharger in an image formation apparatus, and more particularly to a control grid comprising a number of metal thin wires uniformly stretched with respect to a corona discharge electrode and to a method of forming the same. The invention further relates to a method of mounting a corona discharger device so that the above-described control grid is at a proper interval with respect to an image-bearing member in an image formation apparatus.
2. Description of the Prior Art
Chargers or dischargers utilizing corona discharge have been used in image formation apparatus such as electrophotographic copying machines, electrophotographic recording apparatus and the like. Such a corona discharger has its quantity of charge (discharge) greatly affected and varied by environmental conditions such as humidity and the like and therefore, a corona discharger provided with a control grid (hereinafter referred to as the grid) is often used to charge (discharge) to a predetermined potential. In this case, to enhance the control effect of the charging or discharging potential, it is required to narrow the grid interval. Such corona discharger, as described in Japanese Open Patent Application No. 77/76036 (corresponding to U.S. Ser. No. 750,568, is often used to uniformly discharge the surface of a photosensitive medium to 0 potential.
FIG. 1 of the accompanying drawings show, in exploded perspective view, a corona discharger having such a conventional grid. In the Figure, the corona discharger 1 comprises a shield plate 2, discharge wire supporting members 3 integral with the shield plate, a corona discharge wire 4 stretched between the supporting members, and a grid unit 5. In the grid unit 5, metal thin wires are alternately hooked to pins 7.sub.1 and 7.sub.2 provided on support members 6.sub.1 and 6.sub.2 and the thin wires are stretched between the members 6.sub.1 and 6.sub.2 to thereby form a grid 8. Heretofore, when a number of grids with narrowed intervals are to be stretched by the use of such a method of formation, the stretching has been very difficult. In addition, irregularities have been often caused to the interval between the individual thin wires of the grid and to the tension (tensioned state) of the thin wires and it has been difficult to obtain a practical grid. Further, if one of the thin wires is broken by some cause or other, all the other thin wires become loosened or deviated and this has led to an inconvenience that the grid must be re-stretched from the first.
There is also an example of the grid which uses a reticulately woven metal net or a metal plate provided with a number of openings, but in such grid it is more difficult to accurately set up the intervals between the grid openings than in the case of metal thin wires and particularly, when a drum-shaped photosensitive medium is employed, it is difficult to accurately hold the grid cylindrically along the drum surface. Moreover, dust tends to clog the interior of the openings, thus making it difficult to clean the same. Further, to cause the non-uniform potential on the surface of the image-bearing member to be uniformly converged to a predetermined value by the above-described control grid, the interval d.sub.1 between the control grids and the interval d.sub.2 between the control grids and the surface of the image-bearing member become important problems. This is because the charging (discharging) speed and the final surface potential become varied by the ratio between the two intervals, d.sub.1 /d.sub.2.
As regards the former of the intervals d.sub.1 and d.sub.2, it can be considerably accurately set up according to the method as shown in FIGS. 1 and 2. But however accurately the interval d.sub.1 may be set up, it will be difficult to ensure the surface of the image-bearing member to be converged to a predetermined potential, namely, to ensure proper image formation, if the interval d.sub.2 is varied each time the corona discharger device is dismounted with respect to the image formation apparatus for the cleaning of the control grid.
A method of mounting a corona discharger device into an image formation apparatus by inserting the same along a guide rail or by engaging pins with pin holes is heretofore known, but in any of these cases it has been difficult to obtain a high distance accuracy between the control grid and the image-bearing member because of the backlash of the guide rail and the mounting error of the pins in the pin holes.